The present invention relates to a cast steel excellent in workability and quality with few surface flaws and internal defects, having a solidification structure of a uniform grain size, and to a steel material obtained by processing the cast steel.
Further, the present invention relates to a method for processing molten steel capable of improving quality and workability by enhancing the growth of solidification nuclei and fining a solidification structure when producing an ingot or a cast steel from the molten steel after it is subjected to decarbonization refining using a ingot casting method or a continuous casting method.
Yet further, the present invention relates to a method for casting a chromium-containing steel with few surface flaws and internal defects having a fine solidification structure, and to a seamless steel pipe produced using the steel.
Until now, cast steels have been produced by casting molten steel into slabs, blooms, billets and cast strips, etc. through ingot casting methods using fixed molds and through continuous casting methods using oscillation molds, belt casters and strip casters, etc. and by cutting them into prescribed sizes.
Said cast steels are heated in reheating furnaces, etc., and then processed to produce steel sheets and sections, etc. through rough rolling and finish rolling, etc.
Likewise, cast steels for seamless steel pipes are produced by casting molten steel into blooms and billets using ingot casting methods and continuous casting methods. Said cast steels are heated in reheating furnaces, etc., are then subjected to rough rolling, and are sent to pipe manufacturing processes as steel materials for pipe manufacturing. Further, the steel materials are formed into rectangular or round shapes after being heated again, and then are pierced with plugs to produce seamless pipes.
Solidification structures of cast steels before processing, as well as the conditions of processing such as rolling, etc., have a great influence on the properties and quality of the steel materials.
In general, the structure of a cast steel is, as shown in FIG. 7, composed of relatively fine chilled crystals in the surface layer cooled and solidified rapidly by a mold, large columnar crystals formed at the inside of the surface layer, and equiaxed crystals formed at the center portion. In some cases, the columnar crystals may reach the center portion.
When coarse columnar crystals exist in the surface layer of a cast steel as mentioned above, tramp elements of Cu, etc. and their chemical compounds segregate at the grain boundaries of the large columnar crystals, resulting in the brittleness of the segregated portions and the generation of surface flaws in the surface layer of the cast steel, such as cracks and dents caused by uneven cooling, etc. As a result, the yield deteriorates due to the increase of reconditioning work such as grinding and scrapping of the cast steel.
When processing the above-mentioned cast steel by rolling etc., since anisotropy of deformation caused by uneven crystal grain size becomes large, deformation behavior in the transverse direction becomes different from that in the longitudinal direction and the defects such as scabs and cracks, etc., are apt to arise. Further, forming properties such as the r-value (drawing index) deteriorate, and/or surface flaws such as wrinkles (in particular, ridging and roping in stainless steel sheets) appear.
In particular, in a stainless steel material in which the appearance is important, surface flaws such as edge seam defects and roping arise, leading to poor appearance and an increase in the edge trimming amount.
Further, when a seamless steel pipe is produced from the above-mentioned cast steel, surface flaws such as scabs and cracks or internal defects such as internal cracks, voids and center segregation caused by the cast steel remain in the steel pipe. Moreover, during pipe manufacturing, the above-mentioned defects are promoted by forming and piercing and defects such as cracks and scabs are generated on the inner surface of the steel pipe. This leads to the lowering of the yield due to the increase of reconditioning such as grinding or the frequent occurrence of scrapping.
This tendency appears markedly in ferritic stainless seamless pipes containing chromium.
When coarse columnar crystals and large equiaxed crystals exist at the interior of a cast steel, internal defects, such as internal cracks resulted from strain imposed by bulging and straightening, etc., center porosity resulted from the solidification contraction of molten steel and center segregation caused by the flow of unsolidified molten steel at the last stage of solidification, are generated in the cast steel.
Thus the surface flaws generated on a cast steel cause the deterioration of yield caused by an increase in reconditioning work such as grinding and the frequent occurrence of scrapping. If this cast steel is used as it is for processing such as rough rolling and finish rolling, etc., in addition to the surface flaws generated on the cast steel, internal defects such as internal cracks, center porosity and center segregation, etc., remain in the steel material, resulting in the rejection by UST (Ultrasonic Test), the degradation of strength or the deterioration of appearance, and consequent increase of reconditioning work and frequent occurrence of scrapping of the steel material.
Surface flaws and internal defects in a cast steel can be suppressed by improving the solidification structure of the cast steel.
Further, the generation of surface flaws such as surface cracks and dents caused by uneven cooling and uneven solidification contraction arising in a cast steel can be suppressed by making the solidification structure of the cast steel uniform and fine.
Moreover, the generation of internal defects such as internal cracks, center porosity and center segregation, etc., caused by the solidification contraction and the flow of unsolidified molten steel, etc. at the interior of the cast steel can be suppressed by raising the equiaxed crystal ratio at the interior of the cast steel.
Therefore, to suppress the occurrence of surface flaws and internal defects of a cast steel and a steel material produced therefrom and improve the workability and quality such as toughness, etc., of the cast steel, it is important to suppress the coarsening of columnar crystals at the surface layer of the cast steel, to raise the equiaxed crystal ratio at the interior of the cast steel, and to make a uniform and fine solidification structure as a whole.
To cope with these problems, various measures for preventing the occurrence of surface flaws and internal defects in a cast steel and a steel material produced therefrom, such as to devise the form of inclusions in molten steel and to make a solidification structure into fine equiaxed crystal structure by controlling solidification process, have been attempted.
By the way, as measures to raise an equiaxed crystal ratio in the solidification structure of a cast steel, known are (1) a method for casting at a low temperature by lowering the temperature of molten steel, (2) a method for electromagnetically stirring molten steel in solidification process, and (3) a method for generating oxides and inclusions in molten steel by adding themselves or other components in molten steel to act as solidification nuclei at the time of the solidification of molten steel, or a method combining the above methods (1) to (3).
As an embodiment related to low temperature casting by the above method (1), for example, disclosed is a method in Japanese Examined Patent Publication No. 7-84617 for preventing ridging from occurring on a ferritic stainless steel sheet by extracting a cast steel while cooling it in a mold and maintaining the superheat temperature (a temperature obtained by subtracting liquidus temperature of molten steel from actual temperature of molten steel) at not more than 40xc2x0 C. while continuously casting molten steel, and by maintaining the equiaxed crystal ratio of the cast steel to not less than 70%.
However, according to the method disclosed in Japanese Examined Patent Publication No. 7-84617, since the superheat temperature is lowered, there occur the problems of generating nozzle clogging caused by the solidification of molten steel during casting, making casting difficult due to the adhesion of skull, preventing the floating of inclusions caused by the increase of viscosity, and generating defects caused by inclusions remaining in molten steel. Therefore, by this method, it is difficult to lower the superheat temperature to the extent that a cast steel with sufficient equiaxed crystal ratio can be obtained.
Thus, it has not so far been clarified as to how large grain size of equiaxed crystals from the surface layer to the interior of a cast steel is desirable and how uniform the solidification structure should be.
In Japanese Unexamined Patent Publication No. 57-62804, a method is disclosed for reducing a cast steel and bonding the central area with pressure under the condition that unsolidified portions remain in the interior, in order to eliminate internal defects such as center porosity, etc. in the cast steel.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 57-62804, since the center area of a cast steel is bonded with pressure by reduction, when the unsolidified portion is large, the brittle solidified layer is subjected to a large reduction force, and this causes internal cracks and center segregation, etc. On the other hand, when the reduction is insufficient, there are problems that internal defects such as center porosity, etc. remain, and this causes the generation of defects on inner surface, such as cracks and scabs, when the cast steel is pierced in the pipe manufacturing process, which causes the deterioration of quality of the steel pipe.
As mentioned above, by those conventional methods, it is difficult to produce a chromium-containing cast steel having a fine solidification structure and controlled surface flaws and internal defects and further to produce a pipe without breaking down (applying large reduction to) the continuously cast steel. Moreover, it has not so far been clarified as to what kind of casting and treatment of a cast steel should be carried out for producing stably and industrially a pipe of chromium-containing steel (ferritic stainless steel) without defects.
Further, as a method for applying electromagnetic stirring to molten steel according to the above method (2), for example, as disclosed in Japanese Unexamined Patent Publication Nos. 49-52725 and 2-151354, there is a method for improving the solidification structure of a cast steel by applying electromagnetic stirring to molten steel in a mold or downstream of the mold during a solidification process, promoting the floating of inclusions and controlling the growth of columnar crystals.
However, according to the method disclosed in Japanese Unexamined Patent Publication Nos. 49-52725 and 2-151354, when a stirring flow is imposed on molten steel at the vicinity of a mold by electromagnetic stirring, though the solidification structure of the surface layer portion of a cast steel can become fine, that of the interior of the cast steel cannot become sufficiently fine. On the other hand, when a stirring flow is imposed on molten steel downstream of a mold, though the solidification structure of the interior of a cast steel can become fine, large columnar crystals are formed at is the surface layer portion of the cast steel, and thus it is impossible to make the solidification structures of the interior and surface layer portions of the cast steel fine at the same time.
Moreover, by only imposing a stirring flow on molten steel during solidification process with electromagnetic stirring, it is difficult to obtain a cast steel having a fine solidification structure with a prescribed grain size, and thus the effect of electromagnetic stirring on the fining of a solidification structure is limited.
Further, as a method for applying electromagnetic stirring to molten steel, as disclosed in Japanese Unexamined Patent Publication No. 50-16616, there is a method for preventing ridging by applying electromagnetic stirring to molten steel during a solidification process, cutting the tips of growing columnar crystals, making use of the cut tips of the columnar crystals as solidification nuclei, and controlling equiaxed crystal ratio in the solidification structure of the cast steel to not less than 60%.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 50-16616, since electromagnetic stirring is applied to a cast steel leaving a mold, columnar crystals exist in the surface layer of the cast steel. Thus, on the cast steel, surface flaws such as cracks and dents caused by the columnar crystals occur, and on the steel material processed by rolling, etc., in addition to scabs and cracks, surface flaws such as ridging occur.
Yet further, there are methods, as disclosed in Japanese Unexamined Patent Publication No. 52-47522, for producing a cast steel with a fine solidification structure by installing an electromagnetic stirrer at a point 1.5 to 3.0 m distant from the meniscus in a continuous casting mold and stirring molten steel at a thrust of 60 mmHg, and, as disclosed in Japanese Unexamined Patent Publication No. 52-60231, for producing a steel material not having internal defects such as center segregation and center porosity, etc. by casting molten steel at the superheat temperature of 10 to 50xc2x0 C., also applying electromagnetic stirring to unsolidified layer of a cast steel under casting, and making the solidification structure into fine structure composed of equiaxed crystals.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 52-47522, since growing columnar crystals (a dendrite structure) are suppressed by applying electromagnetic stirring to molten steel during solidifying in a mold, though the solidification structure near the portion where electromagnetic stirring is imposed can become fine to some extent, to make the whole solidification structure of the cast steel fine, there is still a problem that a multistage electromagnetic stirrer is necessary and thus the equipment cost increases. Moreover, the installation of a multistage electromagnetic stirrer is extremely difficult from the viewpoint of space for installation, and thus the method disclosed in Japanese unexamined Patent Publication No. 52-47522 has a limitation in producing a cast steel a whole solidification structure of which is fine.
Further, according to the method disclosed in Japanese Unexamined Patent Publication No. 52-60231, since low temperature casting is applied, there are problems that nozzles clog due to the deposition of inclusions on the inner surface of an immersion nozzle, a skin is formed on the surface of molten steel due to the temperature drop of molten steel in a mold, and thus, in some cases, the operation becomes unstable and the casting operation is interrupted.
As mentioned above, in case of low temperature casting, because the temperature for casting molten steel is lowered, problems occur such as the interruption of casting caused by the clogging of an immersion nozzle used for pouring molten steel in a mold and the decline of casting speed caused by the decrease of the feed amount of molten steel, and thus it is difficult to lower the casting temperature to the extent capable of stably making the solidification structure of a cast steel fine.
Further, in case of using an electromagnetic stirrer, even though electromagnetic stirring is applied locally during the solidification of molten steel, there are drawbacks in that columnar crystals and coarse equiaxed crystals are generated and this causes surface flaws and internal defects, and thus yield deteriorates due to the increase of reconditioning and the frequent occurrence of scrapping and the quality of the steel material also deteriorates due to internal defects such as internal cracks and center porosity, etc.
On the other hand, it may be considered to make a solidification structure fine over the whole cross section of a cast steel by installing a plurality of electromagnetic stirrers at the downstream side of a mold including a meniscus. However, since the degree of fining varies depending on the portion where stirring is applied, it is impossible to stably obtain a fine solidification structure over the whole cast steel. If it is required to obtain a stable and fine solidification structure, the number of electromagnetic stirrers to be installed increases. Since the number of electromagnetic stirrers to be installed is restricted by equipment cost and the configuration of a continuous caster, the installation itself of the required number of stirrers is difficult. In any event, even though a plurality of electromagnetic stirrers are installed, sufficient fining of a solidification structure cannot be obtained.
Moreover, as an embodiment of a method for generating oxides and inclusions in molten steel, which act as solidification nuclei, by adding the oxides or inclusions themselves or other components into molten steel according to the above method (3), for example, disclosed is a method, in Japanese Unexamined Patent Publication No. 53-90129, for making whole solidification structure of a cast steel into equiaxed crystals by adding into molten steel a wire wherein iron powder and oxides of Co, B, W and Mo, etc., are wrapped and applying a stirring flow to the place where the wire melts. However, by this method, the dissolution of the additives in the wire is unstable and sometimes undissolved remainders appear. When undissolved remainders appear, they cause product defects. Even if all the additives in the wire are dissolved, it is extremely difficult to uniformly disperse the additives throughout the entire cast steel from the surface layer to the interior. As a result, the size of the solidification structure becomes uneven which is not desirable. Besides, since the effect of equiaxed crystallization is influenced by the position of an electromagnetic stirrer and the stirring thrust, this method has a drawback of undergoing constraint by conditions related to equipment. A method for adding fine particles of TiN, etc. during casting is disclosed in Japanese Unexamined Patent Publication No. 63-140061. However, this method has the same drawbacks as that of Japanese Unexamined Patent Publication No. 53-90129.
With regard to the effect of generating inclusions which act as solidification nuclei by adding required components in molten steel, for example, a method is generally known to form TiN in molten steel of ferritic stainless steel and to produce equiaxed crystals in the solidification structure (Tetsu to Hagane Vol. 4-S79, 1974, for example). However, to obtain a sufficient effect of equiaxed crystallization by the formation of TiN as mentioned above, as described in above xe2x80x9cTetsu to Hagane,xe2x80x9d it is necessary to increase Ti concentration in molten steel up to not less than 0.15 mass %.
Therefore, to obtain sufficient equiaxed crystallization by the formation of TiN as mentioned above, an increased addition amount of expensive Ti alloy is required, which leads to a higher manufacturing cost. Furthermore, there arise the problems of nozzle throttling caused by coarse TiN during casting and formation of scabs on the product sheet. Besides, since the chemical composition of the steel is restricted in relation to the addition amount of TiN, applicable steel grades are limited.
A means is desired for effectively obtaining a cast steel with a fine equiaxed crystal structure by adding some components in as small amounts as possible, and for that reason, a method to add Mg to molten steel is proposed.
However, since the boiling point of Mg is about 1,107xc2x0 C., lower than the temperature of molten steel and the solubility of Mg in molten steel is almost zero, even if metallic Mg is added to molten steel, most of it is vaporized and escapes away. Therefore, if Mg is added by a usual method, the Mg yield generally becomes very low, and thus it is necessary to devise a means for Mg addition.
The present inventors, during the course of research on Mg addition, have found that the composition of oxides formed after Mg addition is affected by not only the composition of molten steel but also the composition of slag. That is, it has been found that, by only adding Mg to molten steel, it is difficult to form inclusions which have composition acting effectively as solidification nuclei in molten steel.
For example, in Japanese Unexamined Patent Publication No. 7-48616, disclosed is a method for improving Mg yield in molten steel by providing the slag covering the molten steel surface in a container such as a ladle with CaOxe2x80x94SiO2xe2x80x94Al2O3 slag containing MgO adjusted to 3 to 15 wt % and FeO, Fe2O3 and MnO adjusted to not more than 5 wt %, and adding Mg alloy passing through the slag, and also, for improving the quality of a steel material by forming fine oxides of MgO and MgOxe2x80x94Al2O3.
According to the method disclosed in Japanese Unexamined Patent Publication No. 7-48616, since the slag of CaOxe2x80x94SiO2xe2x80x94Al2O3 covers the surface of the molten steel, there is an advantage that the improvement of yield can be expected by suppressing the evaporation of Mg. However, by the method disclosed in Japanese Unexamined Patent Publication No. 7-48616, only the total amount of FeO, Fe2O3 and MnO in slag covering molten metal is specified to be not more than 5 wt % and the amount of SiO2 is not specified. Then, if SiO2 is abundantly contained in slag, when metallic Mg or Mg alloy is added, Mg reacts with SiO2 contained in slag and the Mg yield in molten steel drops. When the Mg yield is low, Al2O3, etc., in molten steel can not be reformed into oxides containing MgO, coarse oxides of Al2O3 remain in molten steel and this causes the generation of defects in a cast steel and a steel material after all.
Since the function of Al2O3 system oxides as solidification nuclei is limited, the solidification structure of a cast steel coarsens and defects, such as cracks, center segregation and center porosity, etc., arise on the surface or in the interior of the cast steel, and thus the yield of the cast steel deteriorates.
Further, there are problems that, in the steel material produced from the above cast steel too, surface flaws and internal defects caused by a coarse solidification structure arise, and thus yield and quality deteriorate.
Moreover, since no restrictions are specified for CaO concentration in slag or Ca concentration in molten steel, in some cases, instead of the generation of high-melting-point MgO, etc., low-melting-point complex compounds (CaOxe2x80x94Al2O3xe2x80x94MgO oxides) which do not act as solidification nuclei are generated.
In Japanese Unexamined Patent Publication Nos. 10-102131 and 10-296409, proposed are methods for improving the solidification structure of a cast steel by controlling the amount of Mg contained in molten steel at 0.001 to 0.015 wt %, forming fine oxides with good dispersibility, and distributing the oxides over the entire cast steel.
However, by the methods disclosed in Japanese Unexamined Patent Publication Nos. 10-102131 and 10-296409, since oxides are uniformly distributed from the surface layer portion to the interior of a cast steel at a high density of not less than 50/mm2, in some cases, defects such as cracks and scabs caused by oxides arise on the cast steel, the cast steel being processed or the steel material processed from the cast steel. In this case, reconditioning such as surface grinding, etc. is required or the steel material is scrapped, and thus the yield of products drops.
Further, when oxides are exposed on the surface of a steel material or exist in the vicinity of a surface layer, there are problems that, when the oxides touch acid or salt water, etc., oxides (oxides containing MgO) dissolve out and the corrosion resistance of the steel material deteriorates.
Then, as a result of carrying out various experiments to clarify the optimum conditions for equiaxed crystallization obtained by adding Mg to molten steel, the present inventors have newly found that, even though a molten steel component and/or a slag composition are not changed, the order of the addition of Mg and deoxidation elements such as Al has a great influence on the effect on equiaxed crystallization.
That is, it was found that, when Al is added after Mg is added to molten steel, since Al2O3 covers the surface of MgO generated after Mg addition, the generated MgO does not act effectively as a solidification nucleus.
As a result, the effect of MgO on making a solidification structure fine cannot be obtained, the solidification structure coarsens, and surface flaws such as cracks, etc. and internal defects such as center segregation and center porosity, etc. arise. As a result, reconditioning work of a cast steel and a steel material increases, a cast steel and a steel material are scrapped, and the yield and quality of products deteriorate.
As mentioned above, by conventional methods of adding oxides and inclusions themselves to molten steel as solidification nuclei, and generating solidification nuclei in molten steel by adding a required component, it is difficult to obtain a cast steel of a uniform solidification structure without defects. Therefore, there is a problem that it is impossible to obtain a cast steel with excellent workability during rolling, etc., and further a steel material with good quality and few defects.
It has so far not been clarified as to what kind of solidification structure should be obtained for stably and industrially producing a cast steel with good workability but without defects
As explained above, the reality is that, with the conventional methods for obtaining equiaxed crystallization of a cast steel by casting at a low temperature, adopting electromagnetic stirring or adding oxides which form solidification nuclei, it is impossible to stably and industrially produce a steel material with excellent quality and few defects by suppressing the generation of surface flaws and internal defects such as cracks, dents, center segregation and center porosity, etc which arise in a cast steel, and further obtaining a defect-less cast steel having a solidification structure with a uniform grain diameter, and thus improving the workability of the cast steel.
The present invention has been made in consideration of above circumstances and an object of the invention is to provide a cast steel with excellent workability and/or quality by making a solidification structure fine and uniform and suppressing the generation of surface flaws and internal defects such as cracks, center porosity and center segregation.
Another object of the present invention is to provide a steel material, obtained by processing said cast steel, excellent in workability and/or quality without surface flaws and internal defects.
A further object of the present invention is to provide a method for processing molten steel capable of making a solidification structure of a cast steel fine by promoting the generation of MgO-containing oxides with high melting points and making them act as solidification nuclei.
An even further object of the present invention is to provide a continuous casting method capable of casting a cast steel excellent in quality such as corrosion resistance, etc., with few defects which arise in a steel material during processing the cast steel into the steel material by making the solidification structure of the cast steel fine and suppressing the generation of surface flaws and internal defects such as cracks and segregation, etc.
An additional object of the present invention is to provide a method for casting a cast steel of chromium-containing steel capable of improving product yield, etc., with few defects arising in the steel pipe when a seamless steel pipe is produced from the cast steel by making the solidification structure of the cast steel fine and suppressing the generation of surface flaws and internal defects such as cracks and segregation, etc., and the steel pipe produced from said cast steel.
A cast steel of the present invention complying with aforementioned objects (hereunder referred to as xe2x80x9cCast Steel Axe2x80x9d) is characterized in that not less than 60% of the total cross section of the cast steel is occupied by equiaxed crystals, the diameters (mm) of which satisfy the following formula:
D less than 1.2X1/3+0.75,
wherein D designates each diameter (mm) of equiaxed crystals in terms of internal structure in which the crystal orientations are identical, and X the distance (mm) from the surface of the cast steel.
In a cast steel, by obtaining a solidification structure satisfying the above formula, it becomes possible to make the width of columnar crystals remaining in the surface layer of the cast steel narrow, to enhance resistance to cracking by suppressing micro-segregation caused by the allocation of solid and liquid of molten steel component during solidification, to suppress the generation of crack defects resulted from stress imposed by strain during solidification, bulging and straightening, etc., of the cast steel, and further to prevent the generation of internal defects such as center porosity and center segregation, etc., caused by the solidification contraction and flowing of molten steel in the center portion of the thickness.
Moreover, since Cast Steel A with a solidification structure satisfying the above formula has a uniform deformation property and an excellent workability when processed by rolling, etc., the generation of surface flaws and internal defects are suppressed in the processed steel material.
Further, in Cast Steel A, said equiaxed crystals can occupy the total cross section of the cast steel.
By occupying the total cross section of a cast steel with a uniform and fine solidification structure without columnar crystals and making micro-segregation in the surface layer and interior of the cast steel smaller, the resistance to cracks caused by strain and stress during solidification can be enhanced. As a result, the generation of surface flaws and internal defects of a cast steel can be prevented and workability is improved by the improvement of uniformity of deformation, during forming, over the surface layer to the interior of the cast steel.
Another cast steel with excellent workability of the present invention complying with the aforementioned objects (hereunder referred to as xe2x80x9cCast Steel Bxe2x80x9d) is characterized in that the maximum crystal grain diameter at a depth from the surface of the cast steel is not more than three times of the average crystal grain diameter at the same depth.
By obtaining a solidification structure satisfying above condition regarding crystal grain diameter, the grain diameter of crystals present at a prescribed depth from the surface layer of a cast steel can be uniform. As a result, the local segregation of tramp elements of Cu, etc. at grain boundaries is suppressed and thus grain boundary cracks at the surface layer is also suppressed. Further, when subjected to forming, since uniform deformation of crystal grains can be obtained and the concentration of deformation to specific crystal grains can be suppressed, an r-value, which is a drawing index, can be improved and surface flaws such as wrinkles, ridging and roping, etc., can be prevented.
Further, in Cast Steel B, not less than 60% of the cross section in the direction of the thickness of the cast steel can be occupied by equiaxed crystals.
By occupying not less than 60% of the cross section in the direction of the thickness of a cast steel with equiaxed crystals, it is possible to make the solidification structure of the cast steel into the structure where the growth of columnar crystals is suppressed. As a result, grain boundary segregation in the surface layer and the interior of the cast steel is further suppressed, resistance to cracks caused by strain and stress during solidification is enhanced, the generation of surface flaws and internal defects in the cast steel is suppressed, the isotropy of deformation behavior during forming (stretch to transverse and longitudinal directions by reduction) improves, and thus workability improves. That is, in a steel material, surface flaws such as cracks, scabs and wrinkles caused by the unevenness of deformation by forming, etc., can be prevented from occurring.
Further, in Cast Steel B, the whole cross section in the direction of the thickness of the cast steel can be occupied by equiaxed crystals.
In such a solidification structure, since micro-segregation is further suppressed and a more uniform solidification structure is obtained, for a cast steel, resistance to cracks, etc. is enhanced, the generation of surface flaws and internal defects is more securely prevented, uniformity of deformation from the surface layer to the interior of the cast steel during forming improves, and thus workability, r-value and toughness improve.
A cast steel with excellent quality and workability of the present invention complying with the aforementioned objects (hereunder referred to as xe2x80x9cCast Steel Cxe2x80x9d) is characterized by containing not less than 100/cm2 of inclusions whose lattice incoherence with xcex4-ferrite formed during the solidification of molten steel is not more than 6%.
Inclusions whose lattice incoherence with xcex4-ferrite is small act as inoculation nuclei efficiently generating many solidification nuclei. If many solidification nuclei are formed, a solidification structure becomes fine and, as a result, micro-segregation in the surface layer and the interior of a cast steel is suppressed and crack resistance against uneven cooling and contraction stress, etc. improves. Further, solidification nuclei provide pinning action (suppressing crystal grain growth immediately after solidification) after solidification, the coarsening of a solidification structure is suppressed, and a more stable and fine solidification structure can be obtained.
Thus, a cast steel with such solidification structure transforms easily in the direction of reduction when subjected to forming such as rolling, etc. That is, this cast steel has extremely high workability.
When the number of inclusions contained in a cast steel becomes less than 100/cm2, the number of generated solidification nuclei falls and, at the same time, a pinning action after solidification becomes insufficient, and thus the solidification structure of the cast steel becomes coarse, and, as a result, surface flaws and internal defects arise in the cast steel.
Further, in Cast Steel C, not less than 100/cm2 of inclusions, the sizes of which are not more than 10 xcexcm, can be contained.
If inclusions are fine, since solidification nuclei can be generated efficiently and abundantly and a pinning action can be promoted, a finer and more uniform solidification structure can be obtained. In a cast steel with such a solidification structure, workability is good when subjected to processing such as rolling, etc., and surface flaws and internal defects such as scabs, surface cracks and wrinkles, etc., are not generated in the steel material.
If the size of inclusions exceeds 10 xcexcm, though they act as solidification nuclei when molten steel solidifies, there is a problem that scabs and slivers are apt to arise.
Cast Steel C may be of a steel grade whose solidified primary crystals are composed of xcex4-ferrite.
Even though Cast Steel C is of a steel grade wherein phase transformation occurs during the cooling of the cast steel and structure other than ferrite is formed after solidification or during cooling, inclusions in the Cast Steel C act as inoculation nuclei and promote the generation of solidification nuclei of xcex4-ferrite, and therefore fine and uniform solidification structure can be obtained. As a result, the crystal structure of the cast steel after cooling can be fine.
A cast steel, with the excellent quality of the present invention complying with the aforementioned objects (hereunder referred to as xe2x80x9cCast Steel Dxe2x80x9d) is characterized in that, in said cast steel cast by adding metal or metallic compound to molten steel for forming solidification nuclei during the solidification of the molten steel, the number of the metallic compounds the sizes of which are not more than 10 xcexcm contained further inside than the surface layer portion of said cast steel is not less than 1.3 times the number of the metallic compounds the sizes of which are not more than 10 xcexcm contained in said surface layer portion.
As mentioned above, in Cast Steel D, among the metallic compounds produced by adding metal to molten steel or metallic compounds added directly to molten steel, the metallic compounds the sizes of which are not more than 10 xcexcm are included more abundantly in the interior than in the surface layer portion of the cast steel. These metallic compounds act as solidification nuclei when molten steel solidifies, and reduce the diameter of equiaxed crystals, and, as a result, suppress grain boundary segregation. Further, these metallic compounds provide a pinning action and suppress the coarsening of equiaxed crystals after solidification.
After all, in Cast Steel D, cracks by strain and stress during solidification and surface flaws caused by dents and inclusions are prevented from occurring, resistance to internal cracks caused by strain imposed by bulging and straightening of the cast steel is intensified, and the generation of internal defects such as center porosity and center segregation, etc., caused by solidification shrinkage and flowing of molten steel at the last stage of solidification, is also suppressed.
Besides, in Cast Steel D, since the number of metallic compounds in the surface layer portion is controlled to be less than the number of metallic compounds in the interior portion, when the cast steel is subjected to processing such as rolling, etc., surface flaws produced caused by inclusions are reduced, and quality such as corrosion resistance, etc. and workability, etc. improve.
Here, the surface layer portion in Cast Steel D designates the portion in the range between than 10% and 25% away from the surface. If it deviates from this range, the surface layer portion becomes excessively thin and the interior portion having metallic compound abundantly becomes close to the surface layer portion, the number of metallic compounds in the interior portion increases, the solidification structure of the surface layer portion cannot become fine, and defects are apt to be generated by metallic compounds when the cast steel is processed.
Here, lattice incoherence of metallic compound contained in molten steel with xcex4-ferrite formed during the solidification of molten steel may be controlled at not more than 6%.
By doing so, the ability to form solidification nuclei during the solidification of molten steel improves, a much finer solidification structure can be obtained, and the size of micro-segregation in the surface layer portion and interior portion can be decreased to the utmost. Moreover, deformation in the direction of reduction becomes easy and a cast steel excellent in workability and quality can be stably produced.
Further, Cast Steel D can be a ferritic stainless steel.
In Cast Steel D of ferritic stainless steel, a solidification structure which tends to coarsen can easily be made into fine equiaxed crystals.
In the above cast steel of the present invention, xe2x80x9cMgO-containing oxidesxe2x80x9d formed by adding Mg or Mg alloy in molten steel can be included.
By including xe2x80x9cMgO-containing oxidesxe2x80x9d, it is possible to suppress the aggregation of oxides in molten steel, to raise the dispersibility of the oxides, and to increase the number of the oxides which act as solidification nuclei. As a result, the solidification structure of a cast steel becomes fine more stably.
The aforementioned cast steel of the present invention is, after being heated, for example, after being heated to a temperature of 1,100 to 1,350xc2x0 C., processed into a steel material through rolling, etc. Since the cast steel of the present invention has various characteristics as mentioned above, the cast steel provides the advantages that resistance to cracking during forming such as rolling, etc. is high, the concentration of deformation to specific crystal grains during forming is suppressed, and uniform deformation of crystal grains (isotropy of deformation behavior) can be obtained.
Therefore, since the aforementioned cast steel of the present invention uniformly deforms in the transverse and longitudinal directions by reduction, the steel material of the present invention obtained by processing said cast steel has the advantages that surface flaws such as scabs and cracks, etc. and internal defects such as center porosity and center segregation, etc. generated in the steel material are extremely rare. Moreover, the steel material of the present invention has other advantages in that surface flaws and internal defects caused by inclusions are also rare and qualities such as corrosion resistance, etc. are good.
Methods for processing molten steel required for producing the above-mentioned cast steel of the present invention (hereunder referred to as xe2x80x9cProcessing Method of the Present Inventionxe2x80x9d) will be explained hereafter.
A Processing Method of the Present Invention (hereunder referred to as xe2x80x9cProcessing Method Ixe2x80x9d) is characterized by controlling the total amount of Ca in molten steel refined in a refining furnace at not more than 0.0010 mass %, and then adding a prescribed amount of Mg therein.
By Processing Method I, the generation of calcium aluminate (low-melting-point inclusions such as 12CaO-7Al2O3) can be suppressed. As a result, the generation of ternary system complex oxides of CaOxe2x80x94Al2O2xe2x80x94MgO formed by adding Mg oxides (MgO) to calcium aluminate is prevented and high-melting-point oxides such as MgO and MgOxe2x80x94Al2O3, etc. which act as solidification nuclei can be formed.
Here, the total amount of Ca is the sum total quantity of Ca existing in molten steel and the Ca portion of xe2x80x9cCa-containing chemical compoundsxe2x80x9d such as CaO, etc. The content of Ca specified in Processing Method I means that Ca is not included in molten steel at all or that not more than 0.0010 mass % of Ca is included in molten steel.
Further, in Processing Method I of the present invention, complex oxides of calcium aluminate may not be contained in molten steel.
By doing so, when oxides (MgO) exist in molten steel, the generation of ternary system complex oxides of CaOxe2x80x94Al2O3xe2x80x94MgO generally formed from calcium aluminate and oxides (MgO) is stably prevented, and, as a result, high-melting-point oxides (hereunder occasionally referred to as xe2x80x9cMgO-containing oxidesxe2x80x9d) such as MgO and MgOxe2x80x94Al2O3, etc., can be steadily generated in molten steel, the solidification structure of the cast steel becomes fine, and the generation of surface flaws and internal defects in the cast steel can be prevented.
It is desirable that the addition amount of Mg in molten steel be 0.0010 to 0.10 mass %.
If the addition amount of Mg is less than 0.0010 mass %, the number of solidification nuclei by MgO-containing oxides in molten steel falls and a solidification structure cannot be made fine. On the other hand, if the addition amount of Mg exceeds 0.10 mass %, the effect of making fine the solidification structure is saturated, the Mg and Mg alloy added are ineffective, and also defects caused by the increase of oxides including MgO and MgO-containing oxides may arise.
In a cast steel of the present invention produced by pouring and cooling molten steel processed by Processing Method I of the present invention in a mold, a solidification structure is fined by fine MgO and/or MgO-containing oxides and the generation of surface flaws, such as cracks and dents, etc., arising on the surface of the cast steel and internal defects such as internal cracks, center porosity and center segregation, etc., is suppressed. Then, when a steel material is produced by processing this cast""steel through rolling, etc., the generation of surface flaws and internal defects in the steel material is prevented, reconditioning and scrapping can be prevented, and thus the product yield and the material properties improve.
Another Processing Method of the Present Invention (hereunder referred to as xe2x80x9cProcessing Method IIxe2x80x9d) is characterized by carrying out a deoxidation treatment by adding a prescribed amount of an xe2x80x9cAl-containing alloyxe2x80x9d to molten steel before adding a prescribed amount of Mg therein.
Processing Method II is a method to add xe2x80x9cAl-containing alloyxe2x80x9d in advance, generate Al2O3 by reacting the xe2x80x9cAl-containing alloyxe2x80x9d with oxygen, MnO, SiO2 and FeO, etc., in molten steel, and after that, form MgO or MgOxe2x80x94Al2O3 generated by the oxidation of Mg on the surface of Al2O3 by adding a prescribed amount of Mg. MgO or MgOxe2x80x94Al2O3 present on the surface of Al2O3 acts as solidification nuclei when molten steel solidifies, because its lattice incoherence with xcex4-ferrite which is solidified primary crystals is not more than 6%. As a result, a solidification structure becomes fine, the generation of surface flaws such as cracks, etc., and internal defects such as center segregation and center porosity, etc., is suppressed, and the deterioration of workability and corrosion resistance is also suppressed.
xe2x80x9cAl-containing alloyxe2x80x9d means a substance containing Al such as metallic Al and an Fexe2x80x94Al alloy, etc., and xe2x80x9cMg addedxe2x80x9d means metallic Mg and a xe2x80x9cMg-containing alloyxe2x80x9d such as Fexe2x80x94Sixe2x80x94Mg alloy and Nixe2x80x94Mg alloy, etc.
Further, in Processing Method II of the present invention, before adding Mg to molten steel, a deoxidation treatment by adding a prescribed amount of a xe2x80x9cTi-containing alloyxe2x80x9d, in addition to a prescribed amount of xe2x80x9cAl-containing alloyxe2x80x9d, may be adopted.
By adding a xe2x80x9cTi-containing alloyxe2x80x9d as described above, it is possible to dissolve Ti as a solid solution in molten steel, to precipitate a part of said Ti as TiN, to let them act as solidification nuclei, further to form MgO or MgOxe2x80x94Al2O3 on the surface of Al2O3 generated by deoxidation, and also to let them act as solidification nuclei Here, a xe2x80x9cTi-containing alloyxe2x80x9d means a substance containing Ti such as metallic Ti and an Fexe2x80x94Ti alloy, etc.
In Processing Method II of the present invention, it is desirable that the addition amount of Mg be 0.0005 to 0.010 mass %.
By adding Mg within this range, MgO or MgOxe2x80x94Al2O3 can form sufficiently on the surface of Al2O3 generated by deoxidation. MgO or MgOxe2x80x94Al2O3 acts sufficiently as solidification nuclei and makes a solidification structure finer when molten steel solidifies.
If the addition amount of Mg is less than 0.0005 mass %, the number of oxides having surfaces whose lattice incoherence with xcex4-ferrite is not more than 6% is insufficient and it is impossible to make a solidification structure fine. On the other hand, if the addition amount of Mg exceeds 0.010 mass %, the effect of making fine a solidification structure is saturated and the cost required for adding Mg becomes high.
Further, in Processing Method II of the present invention, the molten steel can be a ferritic stainless steel.
According to Processing Method II of the present invention, it is possible to make fine a solidification structure of ferritic stainless steel which is apt to coarsen. As a result, cracks and dents generated on the surface of a cast steel, internal cracks, center porosity and center segregation, etc., are suppressed.
In Processing Methods I and II of the present invention, it is desirable to add Mg so that oxides such as slag and deoxidation products, etc. contained in molten steel and oxides produced during the addition of Mg to the molten steel satisfy the following formulae (1) and (2):
17.4(kAl2O3)+3.9(kMgO)+0.3(kMgAl2O4)+18.7(kCaO)xe2x89xa6500xe2x80x83xe2x80x83(1)
(kAl2O3)+(kMgO)+(kMgAl2O4)+(kCaO)xe2x89xa795xe2x80x83xe2x80x83(2),
wherein k designates mole % of the oxides.
By Mg addition, complex oxides such as CaOxe2x80x94Al2O3xe2x80x94MgO, MgOxe2x80x94Al2O3 and MgO, etc. which are oxides whose lattice incoherence with xcex4-ferrite is not more than 6% and act effectively as solidification nuclei can be generated. When molten steel solidifies, these complex oxides act as solidification nuclei, generate equiaxed crystals, and make the solidification structure of a cast steel fine.
The Mg addition can apply to molten steel of ferritic stainless steel.
That is, by adding Mg as described above, it is possible to make fine a solidification structure of ferritic stainless steel which is apt to coarsen and to suppress internal cracks, center porosity and center segregation, etc. generated in a cast steel. Further, in a steel material processed from said cast steel, it is possible to prevent the generation of roping and edge seam defects caused by a coarse solidification structure.
A further Processing Method of the Present Invention (hereunder referred to as xe2x80x9cProcessing Method IIIxe2x80x9d) is characterized by adding a prescribed amount of Mg to the molten steel having the concentrations of Ti and N satisfying the solubility product constant where TiN crystallizes at a temperature not lower than the liqudus temperature of the molten steel.
According to Processing Method III, when a temperature is so high that TiN does not crystallize, xe2x80x9cMgO-containing oxidesxe2x80x9d such as MgO and MgOxe2x80x94Al2O3 with good dispersibility are generated, and then, as the molten steel temperature drops, TiN crystallizes on the xe2x80x9cMgO-containing oxidesxe2x80x9d, disperses in the molten steel, acts as solidification nuclei, and makes fine a solidification structure of a cast steel. Here, the addition of Mg is carried out by adding metallic Mg and xe2x80x9cMg-containing alloyxe2x80x9d such as Fexe2x80x94Sixe2x80x94Mg alloy and Nixe2x80x94Mg alloy, etc.
Here, it is desirable that Ti concentration [%Ti] and N concentration [%N] satisfy the following formula:
xe2x80x83[%Ti]xc3x97[%N]xe2x89xa7([%Cr]2.5+150)xc3x9710xe2x88x926,
wherein [Ti] designates the amount of Ti, [%N] the amount of N, and [%Cr] the amount of Cr, in molten steel in terms of mass %.
In Processing Method III of the present invention, since concentrations of Ti and N contained in molten steel are maintained within a prescribed range and a prescribed amount of Mg is added, it is possible to make generated TiN join with MgO-containing oxides having high dispersibility and to disperse TiN in molten steel stably. This TiN acts as solidification nuclei when molten steel solidifies and makes fine a solidification structure further.
Processing Method III of the present invention demonstrates the effect of making fine a solidification structure even on xe2x80x9cCr-containing ferritic stainless steelxe2x80x9d which is apt to coarsen the solidification structure and can prevent the generation of surface flaws and internal defects in a cast steel and a steel material.
Processing Method III of the present invention is suitable, in particular, for casting ferritic stainless molten steel containing 10 to 23 mass % of Cr.
If Cr content is less than 10 mass %, the corrosion resistance of a steel material deteriorates and desired fining effect cannot be obtained. On the other hand, if Cr content exceeds 23 mass %, even though Cr ferroalloy is added, the corrosion resistance of a steel material does not improve, the addition amount of ferroalloy increases, and thus the production cost becomes high.
An even further Processing Method of the Present Invention (hereunder referred to as xe2x80x9cProcessing Method IVxe2x80x9d) is characterized by containing 1 to 30 mass % of oxides reduced by Mg in slag covering molten steel.
According to Processing Method IV, since total amount of oxides contained in slag is maintained at a prescribed value, it is possible that Mg added to molten steel increases the proportion (yield) of Mg which forms MgO and oxides containing MgO and, as a result, it is possible to make fine MgO or oxides containing MgO (hereunder referred to as xe2x80x9cMgO-containing oxidesxe2x80x9d) disperse in molten steel.
Then MgO or MgO-containing oxides act as solidification nuclei and make fine the solidification structure of a cast steel. As a result, it is possible to decrease cracks and dents generated on the surface and cracks, center segregation and center porosity, etc., generated in the interior of a cast steel, to eliminate the necessity of reconditioning a cast steel, to prevent scrapping down, thus to improve the yield of a cast steel, and further to improve the quality of a steel material produced from the cast steel through processing such as rolling, etc.
Here, the above mentioned oxides in slag mean one or more of FeO, Fe2O3, MnO and SiO2.
By properly selecting oxides in slag, it is possible to suppress the consumption of Mg by the oxides in slag, thus to raise Mg yield, and to add Mg to molten steel efficiently.
Further, in Processing Method IV of the present invention, it is desirable that the amount of Al2O3 contained in molten steel be 0.005 to 0.10 mass %.
By doing so, it is possible to make Al2O3 of high melting point into complex oxides such as MgOxe2x80x94Al2O3, etc., to uniformly disperse the complex oxides in molten steel by making use of the dispersibility of MgO, and to raise the ratio of MgO-containing oxides which act as solidification nuclei.
A yet further Processing Method of the Present Invention (hereunder referred to as xe2x80x9cProcessing Method Vxe2x80x9d) is characterized by controlling the activity of CaO in slag which covers molten steel at not more than 0.3 before adding a prescribed amount of Mg to the molten steel.
According to Processing Method V, by adding Mg to molten steel, it is possible to generate, while fining, MgO excellent in lattice coherence with xcex4-ferrite and MgO-containing oxides with high melting point and to disperse them in molten steel.
Then, when molten steel solidifies, since the MgO and MgO-containing oxides act as solidification nuclei, the solidification structure of a cast steel becomes fine.
If the activity of CaO in slag exceeds 0.3, low-melting-point oxides containing CaO which do not act as solidification nuclei or oxides whose lattice incoherence with xcex4-ferrite exceeds 6% increase
In Processing Method V of the present invention, it is desirable that the basicity of slag be not more than 10.
If the basicity of slag is adjusted to not more than 10, it is possible to stably suppress the activity of CaO in the slag and to prevent MgO-containing oxides from converting to low-melting-point oxides or oxides whose lattice incoherence with xcex4-ferrite exceeds 6%.
Further, Processing Method V of the present invention can appropriately apply to molten steel of ferritic stainless steel.
If Processing Method V of the present invention is applied to processing molten steel of ferritic stainless steel, it is possible to make fine a solidification structure which is apt to coarsen when the molten steel solidifies and to prevent surface flaws and internal defects from arising in a cast steel and a steel material produced therefrom.
The above-mentioned cast steel of the present invention can be produced by a continuous casting method and the continuous casting method is characterized by pouring molten steel containing MgO or MgO-containing oxides in a mold and casting the molten steel while stirring it with an electromagnetic stirrer.
By the continuous casting method, it is possible to form MgO and/or MgO-containing oxides with high dispersibility in molten steel and to make fine the solidification structure of a cast steel by the action for promoting the generation of solidification nuclei and the pinning action (suppressing the growth of a structure immediately after solidification) of said oxides.
Moreover, it is possible to reduce oxides present in the surface layer portion of a cast steel by the agitation of an electromagnetic stirrer, and in a cast steel and a steel material, to prevent scabs and cracks, generated by oxides, from occurring, and also to improve corrosion resistance.
Here, in the continuous casting method of the present invention, it is desirable to install an electromagnetic stirrer at a position between the meniscus in a mold and a level 2.5 m away therefrom in the downstream direction.
If an electromagnetic stirrer is installed in said range, it is possible to make fine the solidification structure of the surface layer portion while flushing away oxides captured in the surface layer portion solidified at the initial stage, to contain MgO and/or MgO-containing oxides abundantly in the interior of the cast steel, and to make the solidification structure finer. As a result, in a cast steel and a steel material, it is possible to prevent scabs and cracks generated by oxides from occurring and also to improve corrosion resistance.
If the position of agitation by an electromagnetic stirrer is above the meniscus (surface of molten steel), the agitation stream cannot be imposed on molten steel efficiently. On the other hand, if the position is more than level 2.5 m away from the meniscus in the downstream direction, there arise the problems that the solidified shell is too thick, oxides in the solidified shell which becomes the surface layer portion increase, and thus corrosion resistance deteriorates.
Further, in the continuous casting method of the present invention, it is desirable that the flow velocity of agitation stream imposed on molten steel by an electromagnetic stirrer is not less than 10 cm/sec.
By doing so, oxides captured in the solidified shell of a cast steel can be removed and cleaned by the flow of molten steel.
If the flow velocity of the agitation stream is less than 10 cm/sec., it is impossible to remove oxides in the vicinity of the solidified shell while cleaning. If the flow velocity of agitation stream is too strong, powder covering the surface of molten steel is entangled and the meniscus in a mold is disturbed. Therefore, it is desirable to set the upper limit of the flow velocity of agitation stream to 50 cm/sec.
Further, it is desirable to install an electromagnetic stirrer so that an agitation stream whirling in the horizontal direction is imposed on the surface of the molten steel in a mold.
By the agitation stream whirling in the horizontal direction, it is possible to remove, while efficiently cleaning, oxides captured in the surface layer portion of a cast steel and to secure fine oxides abundantly in the interior of the cast steel.
The continuous casting method of the present invention can appropriately apply to casting a cast steel from molten steel of ferritic stainless steel.
In particular, the above-mentioned molten steel contains 10 to 23 mass % of chromium and 0.0005 to 0.010 mass % of Mg.
By this method, it is possible to form MgO and/or MgO-containing oxides with high dispersibility in molten steel and to make fine the solidification structure of the cast steel by the action for promoting the generation of solidification nuclei and the pinning action (suppressing the growth of a structure immediately after solidification).
Further, it is possible to decrease surface flaws generated in the surface layer portion of a cast steel and defects such as cracks and center porosity, etc., generated in the interior.
Moreover, when piercing the cast steel after processed, the generation of cracks and scabs on the inner surface of a steel pipe is suppressed and the quality of the steel pipe improves.
If Mg content is less than 0.0005 mass %, MgO in molten steel decreases, solidification nuclei do not grow sufficiently, pinning action weakens, and a solidification structure cannot become fine. On the other hand, if Mg content exceeds 0.010 mass %, the effect of making fine the solidification structure is saturated and a remarkable effect does not appear, and the consumption of Mg and xe2x80x9cMg-containing alloyxe2x80x9d, etc., increases and thus the manufacturing cost increases too. Further, if chromium content is less than 10 mass %, the corrosion resistance of a steel pipe deteriorates and the effect of making fine solidification structure decreases. If chromium content exceeds 23 mass %, the addition amount of chromium increases and thus manufacturing cost increases too.
Here, when applying the continuous casting method of the present invention to the continuous casting of molten steel of ferritic stainless steel, the molten steel may be cast while stirring by an electromagnetic stirrer.
By the stirring, it is possible to divide the tips of columnar crystals formed during solidification and to further make fine the solidification structure of a cast steel by the interaction of the suppression of columnar crystal growth and the solidification nuclei generated by the divided tips.
Further, in case of such application, it is preferable to commence the soft reduction of a cast steel from the time when solid phase rate of the cast steel is in the range of 0.2 to 0.7.
By this soft reduction, it is possible to bond with pressure the center porosity generated by the solidification and shrinkage of unsolidified portions remaining in the interior of a cast steel and to prevent the center segregation, etc. generated by the flowing of unsolidified molten steel.
If the reduction is applied from the time when solid phase fraction is less than 0.2, unsolidified areas are so frequent that bonding effect cannot be obtained even though reduction is applied and cracks may arise in a brittle solidified shell. If the reduction is applied from the time when solid phase fraction is more than 0.7, center porosity does not bond with pressure sometimes. Therefore, a large reduction force is required for bonding center porosity with pressure and a large-sized reduction apparatus is required.
A seamless steel pipe of the present invention complying with the aforementioned objects is produced by pouring in a mold molten steel containing 10 to 23 mass % of chromium and 0.0005 to 0.010 mass % of Mg added therein, and by piercing in a pipe manufacturing process a cast steel continuously cast while being solidified with the cooling by a mold and the cooling by the water spray from cooling water nozzles installed in support segments.
In this steel pipe, since it is produced from a cast steel with a fine solidification structure, the generation of cracks and scabs on the surface and inner surface of the pipe is suppressed during piercing in a pipe manufacturing process, reconditioning such as grinding, etc. is not required, and the quality is good.